Merge "list all valid flags (commands) to soong_ui upon unknown command" am:...

Merge "list all valid flags (commands) to soong_ui upon unknown command" am: b3af19ca am: 996117ca am: cca93ea0

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/1965381

Change-Id: Iddced2df9693dcb7224f7e6e50815e51ad2b67f5